Meet DOCSIS, Part 2: the jump from 2.0 to 3.0

In this installment of our in-depth look at the DOCSIC protocol that powers …

Bandwidth growth and node splits

Broadband is now fast enough that the Web is typically no longer limited by raw bandwidth. Streaming audio and VoIP don’t use that much bandwidth either, and only a few people make a hobby out of filling up external drives with downloads. Nonetheless, data use just keeps going up, and that's because there is one application that still demands as much bandwidth as current broadband networks can provide: streaming video. Apparently, between 8 and 10 in the evening, when broadband networks are operating at peak capacity, 43 percent of all traffic is streaming video. Netflix alone is responsible for nearly half of that, 20 percent, at a rate of 1Mbps per user. The problem with streaming video is that people all tend to watch at the same time—in the evenings—and video streams can’t seamlessly slow down like most other types of data traffic. Usually the streams are encoded at maybe three different bitrates, and then the content must be watched at one of these rates. If the available bandwidth can’t accommodate the selected rate, this means the dreaded “rebuffering” message pops up.

This isn’t great news for broadband ISPs, but then again, in the ISP business growth is the name of the game. The price of wholesale “transit” data traffic has fallen steadily, so even for smaller ISPs, who have to pay to exchange traffic with other, larger ISPs, this is unlikely to be problematic. However, as the same number of users generate more and more bandwidth, it becomes necessary to perform node splits. A “node” is the device that interconnects coax and fiber in a hybrid fiber/coax network. The word "node" is also used for a cable segment connected to such a device. Cable TV works just fine with many thousands of subscribers per node. A certain number of those will also subscribe to the cable broadband service—and a few people will only subscribe to the cable broadband service, without a cable TV subscription. These numbers rise and fall over time, so often the number that’s used is households passed (HHP) per node.

Node A is split into a smaller node A and a new node B

Back to our overloaded node. If we’re lucky, such a node has a number of coax feeds coming in that can easily be split in two fairly equal halves. Maybe there’s a 225 HHP cable coming in from the north, a 150 HHP cable from the south, and a 125 HHP cable coming in from the west. This means a 500 HHP node can be split into a 225 HHP node (north) and a 275 HHP node (south + west). If fibers are available, this can be done in a few hours. But it’s also possible that one coax cable is a 400 HHP segment and the other a 100 HHP segment, and there are no fibers available. In this case, it’s necessary to install a new node and new fibers in a new location closer to the 400 congested households. This is much more expensive. Node splits of various difficulty levels happen routinely in cable networks.

IPv6

DOCSIS 3.0 supports both IPv4 and IPv6. Cable operator Comcast is a well-known IPv6 proponent—but not for the most obvious reason. Comcast has so many cable broadband subscribers that even the 10.0.0.0 range of private IPv4 addresses (which holds nearly 17 million of them) isn’t large enough to give all the cable modems in their network a management address. (The management address is a separate, hidden address that the cable modem has for management purposes. It’s unrelated to the IP address(es) the subscriber uses.) DOCSIS 3.0 allows the cable modem to have an IPv6 address while the subscriber has an IPv4 address—or the other way around—as well as both having IPv4 or both having IPv6.

Of course in order to have IPv6 in the cable network, the cable operator must have one or more IPv6 routers to handle the IPv6 traffic. Also, the subscribers and/or the cable modems must be provisioned with IPv6 addresses. With IPv6, subscribers will typically get a range of IPv6 addresses rather than just a single address, like with IPv4. Such ranges or prefixes are given out by DHCPv6 (DHCP for IPv6) servers in a process called “prefix delegation.” A DOCSIS 3.0 CMTS observes which CM gets which IPv6 prefix, so it can forward all IPv6 packets with destinations in the delegated range to the right cable mode.

In conclusion, cable broadband networks are rather complex. But by replacing the backbone of the cable network with fiber coupled with protocol improvements, DOCSIS has been able to provide a lot of bandwidth where consumers need it, while maintaining compatibility with the existing cable TV service. As such, DOCSIS really is one of the unsung heroes of the broadband revolution.

Iljitsch van Beijnum / Iljitsch is a contributing writer at Ars Technica, where he contributes articles about network protocols as well as Apple topics. He is currently finishing his Ph.D work at the telematics department at Universidad Carlos III de Madrid (UC3M) in Spain.